fix: check for go in setup and install via Homebrew if missing

[alohaninja]

Summary

make start (via the setup target) checks for brew, sox/ffmpeg, and whisper-cpp, but never checks for Go itself. On a machine without Go installed, start proceeds to the build step and fails with a cryptic error:

go build -o bin/vox ./cmd/vox
make: go: No such file or directory
make: *** [build] Error 1

Change

• Add a go existence check to the setup target that installs Go via Homebrew when missing, matching the existing pattern used for sox and whisper-cpp.
• Update the README's dependency list to include go.

The check is placed right after the brew guard since Go is required before the build step. Idempotent — skipped when Go is already on PATH.

Validation (local)

• make check-fmt — clean
• make build — succeeds
• make lint (go vet) — clean
• Simulated the missing-go path by masking go from PATH; confirmed the install branch fires (brew install go) and is skipped when Go is present.

Fixes #43

[alohaninja]

Addressed the two README suggestions in 14482cf:

• Requirements line — reframed to macOS + Homebrew as the only true requirements, with go/sox/whisper-cpp noted as auto-installed. Resolves the contradiction with the "handles everything" list.
• Manual setup — added go to brew install go sox whisper-cpp so the manual path doesn't hit the same cryptic build error this PR fixes.

On the Go version nit: leaving existence-only to stay consistent with the sox/whisper-cpp checks — Homebrew installs current Go (1.24+), and Go itself emits a clear version-mismatch error otherwise, so an explicit go.mod version gate would be inconsistent scope here.